Project a. Naturally occurring neuron death is ascribed to a failure of axons to compete for synaptic sites. We shall investigate whether relative age of a neuron influences its chance of survival. Different sets of embryos will be pulse-labelled with tritiated thymidine at day 2 1/2 (stage 17) when the first lumbar motor neurons are born, and at 4 days (stage 23), when the last motor neurons are born. One group of both sets will be sacrificed at day 6, before onset of natural neuron death, and another group at 12 days, after termination of the degeneration period. If older cells have an advantage then the ratio of labelled motor cells counted at 6 and 12 days, respectively, should be higher for older than for younger motor neurons. Project b. An old claim by Hamburger and Levi-Montalcini ('49) that the hyperplasia of spinal ganglia caused by supernumerary limb transplants is due to increased proliferation, has been challenged and will be reinvestigated by autoradiography. Embryos will be pulse-labelled with tritiated thymidine at different stages between days 5 and 8 and sacrificed around day 10. A comparison will be made of counts of labelled cells in the left (control) and the right (hyperplastic) ganglia. Project c. Supernumerary limb transplants are innervated by rostral nerves from the crural plexus. To test the prespecification theory (that motor neurons are prespecified for the innervation of specific muscles) identified muscles in a right transplanted limb will be injected with HRP, whereby the motor neurons in the lumbar column which innervate these muscles will be labelled. In the same embryo, or in other normal embryos, the left homologous muscle will be injected. If the strict prespecification hypothesis is correct, then neurons on identical sites of the left and right motor column should be labelled. If different segments are labelled, then alternative theories will have to be examined.